Conventionally, expansion units (stents) have been proposed to maintain the inside diameter of the luman of a tubular organ so as to prevent, for example, a coronary artery from relapsing into a constricted state after it has been dilated and indwelt by an angiectasia catheter.
A conventional expansion unit, as disclosed in Japanease Patent Examined Publication No. 61-6655, uses a unidirectional shape memory alloy made of Ti-Ni binary alloys. A tube, previously having almost the same inside measurement as that of a normal blood vessel, is shaped before its shape is memorized in the shape memory alloy. After the outside diameter of the shape memory alloy is reduced for easy introduction into a desired location of a blood vessel, the alloy is heated in warm water and the like to be expanded and recovered so as to return to the memorized shape.
(A) For example, in the work of indwelling the above-described conventional expansion unit into a desired location of the blood vessel, the expansion unit is attached to the distal end portion of a catheter and inserted to the desired location in a blood vessel using an X-ray transmission. It is therefore desirable to provide high contrast for X-rays when inserting and indwelling the expansion unit in the desired location of tubular organs, such as a blood vessel.
However, the expansion unit is thinly built because of its inherent functions, such as maintaining the inside diameter of a blood vessel after being indwelt in the blood vessel to keep a stable blood flow. For which reason, poor contrast is provided for the unit.
Further, it is difficult to say from the viewpoint of material that a shape memory alloy, for instance, Ti-Ni binary alloy, comprising the expansion unit provides high contrast.
(B) Also, in the work of indwelling the above-described conventional expansion unit made of the unidirectional shape memory alloy into a desired location in a blood vessel, as a normal practice, a guiding catheter is first dwelt in the vessel and then the expansion unit slided toward a desired location, while simultaneously passing through the inside of the guiding catheter. In this case, however, since the expansion unit slided through the narrow inside of the guiding catheter without a protective wrap around it, problems exist in that the expansion unit may get caught or deformed midway through. Furthermore, because the guiding catheter is generally very rigid, it cannot travel through a bend in the periphery in the blood vessel. Therefore, when a desired location for indwelling is farther away from the bend, the expansion unit may get caught and deformed midway through because the unit, without a protective wrap around it, together with the catheter, slides inside the vessel.
The expansion unit, which has already been proposed by the inventors of this invention, made of bidirectional shape memory alloy, is epoch-making as an expansion unit capable of changing indwelling positions and withdrawal. That is, the expansion unit is characterized in that a bidirectional shape memory alloy, in which two reversible memory shapes, high and low temperature sides, appear reversibly on the borderline of a certain transformation point, is expanded diametrically to obtain the inside measurement of a blood vessel and the like at around body temperature, and is capable of travelling inside of the blood vessel by being contracted diametrically at or below body temperature. However, the above-described expansion unit slides through the narrow inside of the guiding catheter without a protective wrap around it. As a result, such problem exists in that the expansion unit may get caught and deformed midway through. Moreover, while the expansion unit slides inside of the guiding catheter, a large amount of cooling water must be fed to keep the expansion unit contracted diametrically. Further, as is the same with a unidirectional shape memory alloy, the guiding catheter is too rigid to travel through a bend of the periphery in a blood vessel. Therefore, when the expansion unit is inserted after the guiding catheter is removed or when a desired location for indwelling is farther away from the bend, such problems exist that the expansion unit may get caught and deformed midway turough because the unit equipped with the catheter slides through the inside of the blood vessel without a protective wrap around it, and that a large amount of cooling water must be fed to keep the expansion unit contracted diametrically.
It is therefore an object of this invention to provide an expansion unit made of a shape memory alloy, which allows high contrast for X-rays, to insert and indwell the expansion unit into a desired location of a tubular organ.
It is another object of this invention to make it possible for the expansion unit to travel smoothly and readily through the tubular organ.